"Antimicrobial Susceptibility Tests" (AST) are performed to predict the therapeutic effectiveness of different antibiotics for the treatment of infections. The test is performed in vitro (outside the patient) using the microorganism cultivated from an infection site (i.e., wound, urine, stool, etc.). Although the AST method described in the present invention refers to testing bacteria, AST methods have also been developed for other infectious disease agents (such as viruses, fungi, parasites).
Bacterial AST testing is usually performed using a battery of antimicrobial agents. The results provided from this test allow the physician to select the most effective agent from a "menu" of antibiotic choices available. Since therapy is usually initiated before the infectious agent is cultivated, initial antibiotic therapy is selected empirically (i.e., based on the physician's presumptive diagnosis). Therefore, the AST results are, in practice, usually used only to modify therapy, particularly if the patient is not responding well to the initial therapeutic choice.
The most common AST performed today are the broth-dilution Minimum Inhibitory Concentration (MIC) method, and the disk diffusion method. An agar dilution method is also used to a lesser extent..
The broth-dilution method is performed by inoculating a broth growth medium (usually Mueller Hinton Broth) with an organism at a standard density (about 3-7.times.10.sup.5 bacterial cells/milliliter). The broth suspension is then separated into aliquots (usually 0.1 ml) in wells containing antibiotics in a microdilution panel. A series of wells (aliquots) are tested for each antibiotic with each well containing a progressively increasing concentration of drug. The antibiotic/microorganism mixtures are incubated at body temperature (35.degree.-37.degree. C.) for enough time to allow the organism to grow in a well without any antibiotic (growth control well). For inhibitory concentrations of antibiotic(s), no visible evidence of growth is observed after the incubation period. Whereas, growth will be observed in wells were the antibiotic concentrations are not inhibitory. The minimal amount of antibiotic (lowest concentration) needed to inhibit the growth of the organism (the MIC value) is an end point (parameter) used to quantitate the effectiveness of the antibiotic. Lower MIC values indicate more effective antibiotics.
The relative therapeutic effectiveness of the antibiotic is determined by comparing the MIC to the achievable levels of the antibiotic in the body following injection or other delivery methods to the patient. A general "rule of thumb" is that effective agents have an MIC at least 10 times lower than the peak serum level of the antibiotic. In practice, the MIC values are converted to a simpler qualitative result (susceptible, intermediate, or resistant) for the physician. These qualitative values are derived from tables developed by organizations such as the National Committee for Clinical Laboratory Standards (NCCLS).
Background publications on this test include: "Antimicrobial Susceptibility Testing: General Considerations "by James H. Jorgensen and Daniel F. Sahm, Ch. 110, pp. 1277-1280, in Manual of Clinical Microbiology, Sixth Edition, P. R. Murray et. al. eds., ASM Press, Washington D.C., 1995, and "Antibacterial Susceptibility Tests: Dilution and Diffusion Methods" by Gail L. Woods and John A. Washington, Ch. 113, pp. 1327-1341, in Manual of Clinical Microbiology, Sixth Edition, P. R. Murray et. al. eds., ASM Press, Washington, D.C. 1995. These references are incorporated herein by reference.
The Sceptor.RTM. MIC system is very similar to the microdilution method by NCCLS. "Sceptor.RTM. MIC panels" contain antimicrobial agents dried in an 84-well tray. Each panel has up to 83 wells containing antimicrobial agents and one growth control well. A bacterial suspension in Sceptor.RTM. broth medium is used for rehydration of the antimicrobial agents and biochemicals and inoculation of the wells. Following an incubation period, the wells containing the antimicrobial agents are viewed for bacterial growth. The least amount of an antimicrobial which results in no visible growth, is the MIC for that particular antimicrobial.
It had been found that Sceptor.RTM. MIC panels have been exhibiting false susceptibility results with certain resistant microorganisms, such as, for example, for two antimicrobics, Piperacillin and Aztreonam. This necessitated the removal of Piperacillin and Aztreonam from the Sceptor.RTM. panels until a solution for this inconsistency could be found.
The present invention describes a method for solving the problem described above by utilization of certain specific and heretofore unrevealed beneficial concentrations of sugars and/or other ingredients to prevent false susceptibility results in such AST.